# Assumptions in Newton's Second Law

1. Sep 17, 2014

### kq6up

With my understanding of Newton's second -- if I am going to use it to solve a problem, I would need to write ALL velocities in the system with respect to the lab frame (inertial rest frame) -- correct?

Thanks,
Chris

2. Sep 17, 2014

### A.T.

Velocity doesn't even appear in Newton's second law.

3. Sep 17, 2014

### kq6up

It does in the rocket problem. $\dot{p}=\dot{m}v+m\dot{v}=F_{thrust}$. Also, if you are calculating the the change in momentum over some time interval $F=m\frac{\Delta v}{\Delta t}$.

Chris

4. Sep 17, 2014

### A.T.

What, you have there is acceleration and average acceleration, which of course can be expressed in terms of velocity.